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Information About IP Source Guard

IP Source Guard

You can use IP source guard to prevent traffic attacks if a host tries to use the IP address of its neighbor and you can enable IP source guard when DHCP snooping is enabled on an untrusted interface.

After IPSG is enabled on an interface, the switch blocks all IP traffic received on the interface except for DHCP packets allowed by DHCP snooping.

The switch uses a source IP lookup table in hardware to bind IP addresses to ports. For IP and MAC filtering, a combination of source IP and source MAC lookups are used. IP traffic with a source IP address is the binding table is allowed, all other traffic is denied.

The IP source binding table has bindings that are learned by DHCP snooping or are manually configured (static IP source bindings). An entry in this table has an IP address, its associated MAC address, and its associated VLAN number. The switch uses the IP source binding table only when IP source guard is enabled.

IPSG is supported only on Layer 2 ports, including access and trunk ports. You can configure IPSG with source IP address filtering or with source IP and MAC address filtering.

IP Source Guard for
Static Hosts

Note

Do not use IPSG (IP
source guard) for static hosts on uplink ports or trunk ports.

IPSG for static hosts
extends the IPSG capability to non-DHCP and static environments. The previous
IPSG used the entries created by DHCP snooping to validate the hosts connected
to a switch. Any traffic received from a host without a valid DHCP binding
entry is dropped. This security feature restricts IP traffic on nonrouted Layer
2 interfaces. It filters traffic based on the DHCP snooping binding database
and on manually configured IP source bindings. The previous version of IPSG
required a DHCP environment for IPSG to work.

IPSG for static hosts
allows IPSG to work without DHCP. IPSG for static hosts relies on IP device
tracking-table entries to install port ACLs. The switch creates static entries
based on ARP requests or other IP packets to maintain the list of valid hosts
for a given port. You can also specify the number of hosts allowed to send
traffic to a given port. This is equivalent to port security at Layer 3.

IPSG for static hosts
also supports dynamic hosts. If a dynamic host receives a DHCP-assigned IP
address that is available in the IP DHCP snooping table, the same entry is
learned by the IP device tracking table. In a stacked environment, when the
master failover occurs, the IP source guard entries for static hosts attached
to member ports are retained. When you enter the
show ip device tracking all
EXEC command, the IP
device tracking table displays the entries as ACTIVE.

Note

Some IP hosts with
multiple network interfaces can inject some invalid packets into a network
interface. The invalid packets contain the IP or MAC address for another
network interface of the host as the source address. The invalid packets can
cause IPSG for static hosts to connect to the host, to learn the invalid IP or
MAC address bindings, and to reject the valid bindings. Consult the vender of
the corresponding operating system and the network interface to prevent the
host from injecting invalid packets.

IPSG for static hosts
initially learns IP or MAC bindings dynamically through an ACL-based snooping
mechanism. IP or MAC bindings are learned from static hosts by ARP and IP
packets. They are stored in the device tracking database. When the number of IP
addresses that have been dynamically learned or statically configured on a
given port reaches a maximum, the hardware drops any packet with a new IP
address. To resolve hosts that have moved or gone away for any reason, IPSG for
static hosts leverages IP device tracking to age out dynamically learned IP
address bindings. This feature can be used with DHCP snooping. Multiple
bindings are established on a port that is connected to both DHCP and static
hosts. For example, bindings are stored in both the device tracking database as
well as in the DHCP snooping binding database.

IP Source Guard
Configuration Guidelines

You can configure
static IP bindings only on nonrouted ports. If you enter the
ip source bindingmac-addressvlanvlan-id
ip-addressinterfaceinterface-id global configuration command on a
routed interface, this error message appears:

Static IP source binding can only be configured on switch port.

When IP source
guard with source IP filtering is enabled on an interface, DHCP snooping must
be enabled on the access VLAN for that interface.

If you are
enabling IP source guard on a trunk interface with multiple VLANs and DHCP
snooping is enabled on all the VLANs, the source IP address filter is applied
on all the VLANs.

Note

If IP source
guard is enabled and you enable or disable DHCP snooping on a VLAN on the trunk
interface, the switch might not properly filter traffic.

You can enable
this feature when 802.1x port-based authentication is enabled.

Configuring IP
Source Guard for Static Hosts on a Layer 2 Access Port

You must configure
the
ip device trackingmaximumlimit-number interface configuration command
globally for IPSG for static hosts to work. If you only configure this command
on a port without enabling IP device tracking globally or by setting an IP
device tracking maximum on that interface, IPSG with static hosts rejects all
the IP traffic from that interface.

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